Building monitoring and control systems including security system, HVAC and other monitoring and control systems are in increasing use in both commercial buildings and residential dwellings. For security systems, the increasing use is due in part to a long-term perception of increasing crime rates along with increasing awareness of the availability of building monitoring and security systems. For HVAC systems, the increasing use is due in part to the desire to reduce heating and cooling costs, and to save energy.
A building monitoring and/or control system typically includes a variety of remote units coupled to detection devices and at least one master unit which typically resides in a central location in the building and can include annunciation functions and reporting functions to another location such as a central reporting service or police department. Remote units have, in the past, been hard wired to the master unit. For example, in a security system, reed switches or Hall effect switches are often disposed near magnets located near doors and door jambs, with a door opening making or disrupting continuity, with the resulting signal being received by the master unit.
In hardwired systems the remote units and the detection devices may be nearly one in the same. For example, the detection device may be a foil trace on a glass pane and the remote unit may be wire terminals with optional signal conditioning equipment leading to a wire pair connected to the master unit. Hard wired units can be installed most easily in new construction, where running wire pairs is easier than in existing buildings. Installing hard-wired systems can be very expensive in existing buildings due in part to the labor costs of snaking wires through existing walls and ceilings. In particular, on a point-by-point basis, retrofitting residential dwellings can be expensive because houses are often not designed to be continually changed, as are many office buildings. For example, most houses do not have dropped ceilings and utility closets at regular intervals. Houses can have higher aesthetic expectations than commercial office buildings, requiring greater care in installing and concealing wiring.
Wireless security systems have become increasingly common. Existing systems use radio frequency transmission, often in the 400 MHz region. Wireless systems can greatly reduce the need for wiring between remote and master unit or units. In particular, wireless systems can communicate between the remote units and the master units without wiring. Remote units still require power to operate, and can require wiring to supply that power, which can add a requirement for power wiring where the power had been provided in hard wired systems over the wiring used to communicate between remote units and the master unit. The power requirement can partially negate the wireless advantage of radio frequency units, as some wiring is still required. The power supply wiring requirement is often eliminated with the use of batteries. Battery life is largely a function of power consumption of the remote units. The power consumption is dependent upon both the electronics and upon the duty cycle of the unit.
Current wireless systems typically utilize remote units which can only transmit and master units which only receive. Remote units often transmit sensor data for needlessly long periods, and at higher power than is required, as there is no bi-directional capability, and therefore no way for the master unit to acknowledge receipt of the first remote unit message, or a low power message. Sometimes, the remote units transmit a health status message at regular periodic intervals. The health status message gives the health of the remote unit, sometimes includes sensor data, and informs the master unit that the remote unit is still functioning. The periodic transmissions can be scheduled at the remote units by DIP switches or local programming of the remote units, but typically cannot be adjusted by the master unit as the communication between master and remote is unidirectional and the master has no way to adjust the timing of transmissions of the remote units. Since there is no coordination between the transmission times of the remote units, collisions can occur between remote unit transmissions, which may reduce the overall reliability of the system. To increase the probability that a particular remote unit transmission is received by the master, the remote unit may make the same transmission many times. However, this can significantly increase the power consumed by the remote units.
Another limitation is that false alarms can be generated. False alarms undermine the credibility of real alarms and can cost money to respond to. For security systems, private security firms often charge to investigate alarms reported to them. Many municipalities charge large fees for false alarms that are reported to police departments. Too many false alarms can result in all or part of a security system to be ignored or turned off entirely. For HVAC systems, false alarms can cause, for example, heat to be applied even if it is not desired. As can be seen, this can decrease the efficiency of the HVAC system.
What would be desirable, therefore, is a bidirectional wireless security, HVAC or other building monitoring system that allows communication between the master and remote units for increased reliability. What would also be desirable is a system that has one or more low power modes for conserving valuable power resources.